With continuous scaling down of semiconductor devices, short channel effects are becoming more significant. Thus, a gate stack configuration comprising a high-K gate dielectric and a metal gate conductor is proposed. To avoid degradation of the gate stack, semiconductor devices with such a gate stack configuration are manufactured generally by means of the replacement gate process. The replacement gate process involves filling the high-K dielectric and the metal gate conductor in a gap defined between gate spacers. However, it is becoming more and more difficult to fill the high-K dielectric and the metal gate conductor in the small gap due to the scaling down of the semiconductor devices.